TDA7250 feedback compensation question

[sendler]

Any help with the circuit theory of the feedback loop of the TDA7250 please?
R1=22k, R2=1k5 sets the feedback to the -input at pin 20. What is the function of R3 560R through C3 1200pf back to the +input at pin2?
What effects on gain and stability would occur as the input termination R4 (22k to ground on pin2) changes from 100k to 100R with the use of a shunt style passive attenuator on the input? Can the network of R3/C3 function with a wide changing R4? If I decided to change the value of R1 to 4k and R2 to 274 to minimize the parallel reduction of R2 by the low R4 (100R at at low volumes), would C1 increase to 47pf? What changes would be needed to rebalance R3/C3. Or, should I just forget trying to run straight in to the 7250 and use an op amp in front of it to buffer the volume control away from the amp circuit.

[sendler]

This kit is a good deal with the heat sink.
http://store.qkits.com/moreinfo.cfm/K4005B

[pacificblue]

Originally posted by sendler
What is the function of R3 560R through C3 1200pf back to the +input at pin2?

RF-filter, so that your amplifier does not work as FM tuner.

Originally posted by sendler
What effects on gain and stability would occur as the input termination R4 (22k to ground on pin2) changes from 100k to 100R with the use of a shunt style passive attenuator on the input?

R4 determines the input impedance, i. e. the load your source sees. 100 Ohm could be too low for the output filter of your source. A lot of the audible frequency range would be filtered out.

Originally posted by sendler
If I decided to change the value of R1 to 4k and R2 to 274 to minimize the parallel reduction of R2 by the low R4 (100R at at low volumes), would C1 increase to 47pf? What changes would be needed to rebalance R3/C3. Or, should I just forget trying to run straight in to the 7250 and use an op amp in front of it to buffer the volume control away from the amp circuit.

The easiest solution would be to replace R4 with an attenuator of the same value (or smaller like 10 or 15k) and leave everything else as it is.

And have this thread moved to the chipamp section.

[sendler]

Originally posted by pacificblue

RF-filter, so that your amplifier does not work as FM tuner.


R4 determines the input impedance, i. e. the load your source sees. 100 Ohm could be too low for the output filter of your source. A lot of the audible frequency range would be filtered out.
.
The easiest solution would be to replace R4 with an attenuator of the same value (or smaller like 10 or 15k) and leave everything else as it is.

And have this thread moved to the chipamp section.

.
The schematic as shown already has a 20k shunt attenuator on the input. 20k fixed in series, R4 to ground changes from 100k to 100R. The input signal will see 20k to 100k. Never less than 20k. Can R1/R2 and R3/C3 function properly with R4=100R?

.
This is not really a chip amp like the 3886 as the high current is all external but I may try over there as well.
.

[pacificblue]

Originally posted by sendler
The schematic as shown already has a 20k shunt attenuator on the input. 20k fixed in series, R4 to ground changes from 100k to 100R. The input signal will see 20k to 100k. Never less than 20k. Can R1/R2 and R3/C3 function properly with R4=100R?

R4 won't affect the RF-filter R3/C3.

Originally posted by sendler
This is not really a chip amp like the 3886 as the high current is all external but I may try over there as well.

There have been discussions, whether those driver ICs belong to the solid state or chipamp section. They ended up in the chipamp forum.